Cardiovascular disease is the leading cause of death in the developed world. Epidemiological studies have shown the profound importance that the prenatal period has on life-long health. A likely explanation for this relationship is that the late fetal period, a period of robust growth, determines the number of cardiomyocytes and the structure of the coronary vasculature for life. Adverse intrauterine conditions leading to fetal growth restriction alter cardiac load and compromise heart growth. While the ability of adult heart to adapt to cardiovascular disease is limited, the fetal heart is highly plastic, undergoing rapid adaptation to best meet intrauterine conditions. The number of fetal cardiomyocytes and the anatomical dimensions of the fetal coronary circulation are not fixed but can be changed, both up and down, by prenatal conditions. Unfortunately, fetal responses to sub-optimal intrauterine environments may not lead to favorable outcomes in the fetus or later in the adult. It will not be possible to understand the devastating effects of intrauterine growth retardation on the fetal myocardium unless we deliberately study cardiomyocyte and coronary microvascular growth in a well controlled experimental load model. The overarching hypothesis of the proposal is that fetal heart growth is exquisitely sensitive to hemodynamic load and that altered growth leads to irreversible changes in cardiac myocyte and coronary vascular function for life. The proposed experiments will demonstrate for the first time the precise responses that determine how the fetal heart adapts to changes in loading conditions during late gestation when all the terminal maturation processes are underway. Completion of the proposed work will provide a clearer understanding of the relationship between fetal cardiomyocyte and coronary vascular growth, how this interrelationship affects fetal cardiac function, which changes persist into adulthood and how adult heart function is affected. This work is an essential step in understanding the mechanisms responsible for the association between fetal growth restriction and adult cardiovascular disease.